a random stream is fundamentally incompressible, but for any compression algorithm failure to
compress does not imply randomness. Many PRNGs and all CSPRNGs produce output that doesn't
compress, but the output is completely deterministic containing no more entropy than the PRNG
state. Proving randomness is hard. 7z failing to compress something is basically uninteresting.
For a hardware random source I'd first think about using statistical tests like DIEHARD to search
for correlations between samples.
Quality hardware random sources have to continuously monitor themselves for faults (loose
connection could otherwise cause the output to drop to all zeroes thereafter).
BTW an SPI ADC uses fewer pins and is still fast.